Bring a large pan of water to the boil and cook the macaroni for 5 minutes, then add the cauliflower florets and cook for a further 3 minutes. Drain well and return to the pan.
Melt the butter in a medium saucepan and add the flour. Beat together until you have a smooth paste. Remove from the hob and gradually pour in the milk, stirring continuously with a wooden spoon until smooth. Return to the heat and bring to a simmer, stirring until the mixture is thick enough to coat the back of the spoon.
Stir in three-quarters of the grated cheddar along with the mustard and chives until well combined. Pour the cheesy sauce over the cooked macaroni and cauliflower, add the spring onions and mix well.
Preheat the grill to high. Spoon the macaroni mixture into a 2-litre ovenproof dish and scatter the cherry tomato halves over the top. Sprinkle over the breadcrumbs and remaining cheddar and place under the hot grill for about 5 minutes until crisp and golden on top.